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A common problem with mine. Your technique to tighten the rear bolt first is exactly what I do, of course at risk of breaking the adjuster screw if you get too carried away (I now use a pry bar to help after breaking one screw as they can be difficult to source).
By the time you get the A/C belt properly tensioned (I actually use a belt gauge now), what started out as aligned pulleys is now out by a noticeable amount. With a new belt, no problem, all is well, but as it ages it quickly starts to get noisy. New mounting bushings would of course help. One thing I tried way back on a 240 was shimming the compressor/alternator mounting bracket at the block as both pulleys were difficult to keep aligned even with new bushings. It definitely worked, but was a lot of effort.
If belt squeal is an issue, be sure you're using an appropriate width belt (13mm as I recall) that's not too worn or glazed, with the outer edge still riding more or less level with the outer edge of the pulleys (a new belt usually rides slightly higher in the compressor pulley as I recall). As a belt wears, it rides lower in the pulley and as it gets closer to the bottom there is both less surface contact and if it starts to ride on the bottom then there is less pressure to the side, all of which leads to squealing, in addition to belt glazing. When manufacturers cross reference their standard line of SAE belts to metric sizes, they often error to the smaller widths and if they're too narrow they will quickly start to ride lower and get noisy. Overtensioning the belt to keep it quiet not only risks premature belt failure, it's also harder on the accessory shaft bushings. Belt dressing compound helps, but I've found it at best a short term solution. Different brands of belts may behave longer than others depending on the designs and compounds used. The goto belt for many of us is Contitech, if for no other reason it's one of the few makes with an exact metric fitment. Note that turbos and NA engines call for different length A/C belts, but I always use the longer belt in both engines now to make life simple for my parts stock and also the shorter belt is harder to mount.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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